The invention relates to an optical wavelength monitor and for a method for investigating optical signals. The invention is based on a priority application DE 100 50 973.8 which is hereby incorporated by reference.
With the constant demand for transmission capacity in the telecommunications sector (mainly due to the popularisation of the internet) great hopes are placed on the use of frequency division multiplex. Since glass fibres, as are used for the transmission of optical signals, are sufficiently transparent for an extended spectral range, a better utilization rate is achieved if optical frequency division multiplex (OFDM) or wavelength division multiplex (WLDM) is used. Here a number of modulated optical carriers whose frequencies differ are transmitted simultaneously in an optical waveguide (generally glass fibre). This facilitates a considerable increase in the maximum quantity of optical signals transmitted in already existing optical cables without the need to install new cables.
In parallel to this development there has been a constant increase in the number of carrier frequencies employed which can currently amount to more than 250 for one single glass fibre. As such a large number of carrier frequencies inevitably causes the distances between channels defined by the relevant carrier frequencies to be selected to be ever smaller, it is essential to be able to employ a means of monitoring the optical signals transmitted by the carrier frequencies in the course of the transmission links. In general the channels are arranged in a fixed grating, for example in accordance with a standard defined by the ITU in which the distances between adjacent channels, either in terms of wavelength or frequency, are equal. Therefore it is very important that for example a shift or disturbance of some carrier frequencies in their respective channels along the transmission link should be ascertained as early as possible for their optional correction.
U.S. Pat. No. 5,777,763 has disclosed an optical module with which optical signals transmitted in an optical fibre can be investigated along the latter. In particular this optical module can be used to compare the individual carrier wavelengths (inversely proportional to the carrier frequencies) in the respective channels of the transmitted optical signals with reference wavelengths. The optical module basically consists of a central waveguide grating and two planar lenses, one for the input, the other for the output. This optical module is directly inserted into the transmission link to be investigated. An optical input fibre is coupled to the input lens. Similarly the output lens is coupled to an optical output fibre. With the aid of the waveguide grating it is possible to investigate individual carrier wavelengths with which optical signals are transmitted by positioning detectors at specific points of this optical module in order to measure backscattered light in specific channels. It is provided that two detectors be connected for each channel to be investigated. In this way it is possible to measure both the amplitude of the power level in the investigated channels as well as a wavelength shift of the carrier wavelength. The results of such measurements are then used to optionally control the transmitted optical signals by means of feedback. This therefore provides a possibility of being able to correct excessively high attenuations or wavelength shifts.
This optical module according to U.S. Pat. No. 5,777,763 involves the considerable disadvantage of being directly connected to the transmission link. The optical fibre serving as transmission link is thereby interrupted in order to be able to introduce the optical module. There is a substantially increased risk of the transmitted optical signals being negatively influenced by the insertion of a defective optical module.
It is already prior art for similar optical modules for investigating power levels and/or wavelength shifts of transmitted optical signals not to be inserted directly into the transmission link but to be connected to the transmission link with the aid of a coupler (tap coupler), generally a directional coupler, or optionally a splitter. This has the advantage that the transmitted optical signals are not directly influenced. However, this technique has the disadvantage of the too small portion of split-off transmitted optical signals, which considerably impedes the satisfactory investigation of the optical signals.
The object of the invention is to design an optical wavelength monitor with which it is possible to perform very reliable measurements of power levels and/or wavelength shifts of carrier wavelengths in different channels of transmitted optical signals, without too greatly influencing the respective transmitted optical signals.
This object is achieved in accordance with the invention by means of an optical wavelength monitor with an optical coupler for coupling at least a part of the optical signals transmitted in an optical waveguide, which signals are transmitted by means of different carrier wavelengths divided into channels, wherein the coupler is connected via an optical amplifier to a wavelength demultiplexer which selects at least one carrier wavelength and forwards the selected carrier wavelengths to specific outputs to which optical detectors are connected, which each convert an optical signal, detected in a specific channel, into an electrical signal, wherein an oscillator is provided which is suitable for modulating the optical signals, supplied to the optical amplifier, with a frequency.
In accordance with another embodiment of the invention, its object is achieved by a method for investigating optical signals which are transmitted in an optical waveguide and consist of different carrier wavelengths divided into channels, wherein at least one property of these optical signals is measured with the aid of the optical wavelength monitor and is compared with pre-stored calibration values.
The use of an oscillator for methoding the small extracted portion of transmitted optical signals in accordance with the lock-in principle enables these optical signals to be satisfactorily investigated via a coupler. With this construction according to the invention it is also possible to utilize the advantages of the use of a coupler, i.e. the possibility of very minor influence of the transmitted optical signals, without being too limited by the poor ratio of the coupled optical signals relative to the noise. The modulation of the coupled optical signals, advantageously between two values, offers a considerably higher degree of stability of monitoring measurements performed on these optical signals. Additionally the sensitivity of such monitoring measurements is considerably better. The use of an optical wavelength monitor of this type offers the possibility of optionally simultaneously investigating individual or all of the channels used. Furthermore, advantageously it is possible to select between different options for the investigation of the optical signals by differently setting the reference wavelength of the detectors using a controllable temperature source for example. An optical wavelength monitor of this kind facilitates selective measurements of power levels of optical signals or of noise or also of wavelength shifts of these optical signals in specific channels, without the need for different constructions for this purpose.
Advantageous developments of the invention are described in the dependent claims, the following description and the drawings.